A technique for supporting a valve timing control apparatus is disclosed in U.S. Pat. No. 6,176,210B1 (hereinafter referred to as Reference 1). The valve timing control apparatus disclosed in Reference 1 includes a drive-side rotation member integrally rotating with a sprocket to which a rotational driving force of a crankshaft is transmitted via a chain. A bearing portion is integrally formed at the drive-side rotation member. The valve timing control apparatus also includes a driven-side rotation member of which a rotational phase relative to the drive-side rotation member is changed by a control of operation oil. A boss (i.e., a member having a journal at an outer periphery), to which a camshaft is connected, is connected to the driven-side rotation member.
In a case where a surface of the sprocket facing an internal combustion engine (i.e., an engine) is formed to be flat as in Reference 1, the surface may make contact with an outer wall surface of the engine (specifically, a cylinder head) so as to serve as a thrust bearing, thereby supporting the valve timing control apparatus.
A valve timing control apparatus disclosed in JP2009-138611A (hereinafter referred to as Reference 2) includes a drive-side rotation member (i.e., a shoe housing) integrally rotating with a sprocket to which a rotational driving force of a crankshaft is transmitted, and a driven-side rotation member (i.e., a vane rotor) of which a rotational phase relative to the drive-side rotation member is changed by a control of operation oil. The driven-side rotation member is connected to a camshaft.
According to Reference 2, a portion of the camshaft is radially enlarged to form a stepped portion that makes contact with an outer surface of an engine head so that the stepped portion serves as a thrust bearing for supporting the valve timing control apparatus.
The camshaft may be supported in a slightly movable manner relative to an outer wall of an internal combustion engine (i.e. an outer wall of a cylinder head) in a direction along a rotational axis of the camshaft. In a case where the camshaft moves towards the engine, the sprocket makes contact with an outer wall surface of the engine according to the valve timing control apparatus disclosed in Reference 1.
Accordingly, in a case where a wide surface of such sprocket makes contact with the outer wall surface of the engine, the sprocket or the chain may become worn or an abnormal sound may be generated by the contact of the sprocket relative to the outer wall surface of the engine. In addition, when the sprocket or the chain makes contact with the outer wall surface of the engine (cylinder head), a frictional force therefrom directly acts on the crankshaft, which may cause a timing delay of control of the rotational phase of the valve timing control apparatus.
Then, according to Reference 2, a large diameter portion of the camshaft makes contact with the outer wall surface of the engine so that the sprocket or the chain is positively separated from the outer wall of the engine. Alternatively, a reduction of a contact area of the sprocket or the chain relative to the outer wall surface of the engine may be considered. However, in case of processing the camshaft, the process may take more time and be complicated.
Further, in the same way as the large diameter portion of the camshaft in Reference 2, a portion of the driven-side rotation member in the vicinity of a rotational axis thereof may be enlarged to form a large diameter portion that protrudes towards the engine. Then, the large diameter portion makes contact with the outer wall surface of the engine. However, the driven-side rotation member that simply makes contact with the outer wall surface of the engine may cause an abrasion, which leads to a durability issue.
In view of improvement of the durability, a material having a high abrasion resistance may be used for the driven-side rotation member. However, processability and cost may create inconveniences. In addition, in order to improve the abrasion resistance at a portion making contact with the outer wall surface of the engine for improvement of the durability, a surface treatment may be performed on only the contact portion. However, a masking of a range for treatment or a complicated process for the surface treatment may result in less feasibility.
A need thus exists for a valve timing control apparatus which is not susceptible to the drawback mentioned above.